1. Field of the Invention
The present invention relates to a belt tension measuring apparatus for measuring a tension of a belt spanned under tension. More particularly, the invention is concerned with a belt tension measuring apparatus for measuring tension of a belt employed in a power transmission mechanism such as a belt spanned under tension between pulleys of a belt/pulley transmission mechanism of an internal combustion engine for a motor vehicle, although the invention can find other applications.
2. Description of Related Art
For having better understanding of the present invention, description will first be made in some detail of the background techniques thereof. FIG. 4 is a schematic diagram for illustrating a conventional method known heretofore for measuring a tension of a belt constituting a part of a power transmission mechanism of an internal combustion engine for a motor vehicle. Referring to the figure, a belt 43 is spanned over and around pulleys 42a, 42b and 42c. For measuring a tension of the belt 43, the belt 43 is pushed downwardly under a predetermined pressure applied by means of a pressure gauge (or manometer) 44 disposed so as to bear against the belt 43 at a predetermined position thereof, whereby the belt 43 is deflected downwardly by a predetermined distance or deflection .DELTA.L. The pressure P.sub.1 applied to the belt 43 at that time point is measured by using the pressure gauge 44 itself.
Now, the belt tension measuring method will be elucidated. It is practically impossible to measure straightforwardly or directly the tension of the belt in the state spanned between and around the pulleys 42a and 42b. Accordingly, in order to measure the tension of the belt 43 spanning the pulleys 42a and 42b, the pressure gauge 44 is pressed against the belt 43 approximately at a mid or center position thereof under a preset pressure to thereby deflect the belt 43 by a predetermined magnitude. In this state, deflection .DELTA.L of the belt 43 as well as a pressure P.sub.1 read from the pressure gauge are recorded.
Subsequently, the tension T of the belt 43 is estimated in accordance with an expression defining a relation between the pressure P.sub.1 and the deflection .DELTA.L, which relation has previously been determined experimentally by a bench test or the like process.
Further, as an applied technique of the tension measurement for a flexible member such as the belt employed in association with a power transmission such as that of the engine of a motor vehicle, there is known, for example, a string tension measuring technique adopted in a process for tuning a string instrument. FIG. 5 is a block diagram showing schematically an apparatus for measuring a string tension of a musical instrument. A sound generated by vibration of a string of concern is collected by a microphone 1 which converts the sound as caught into an electric acoustic signal which is then outputted to a signal processing unit 2 which serves for processing the acoustic signal supplied from the microphone 1 to thereby measure a natural oscillation or vibration frequency of the string and display the natural (vibration) frequency (also known as the characteristic or proper frequency) in the form of numerical values.
To this end, the signal processing unit 2 is comprised of an input signal shaping circuit 21 for shaping waveform of the acoustic signal inputted from the microphone 1 to thereby eliminate noise components, a frequency counter 22 for sampling or quantizing the acoustic signal outputted from the input signal shaping circuit 21 to thereby convert the input signal into a digital signal for the purpose of frequency measurement thereof, a CPU (abbreviation of Central Processing Unit) 23 for processing the frequency data as measured for the numerical display thereof, and a display drive circuit 25 for displaying the frequency data as processed on a display device 24.
Operation of the string tension measuring apparatus will be explained briefly. When a string of a musical instrument such as violin is caused to vibrate under frictional sweeping of a bow, the string vibrates at a frequency intrinsic to the string, whereby a vibration sound is generated, which sound is collected by the microphone 1 to be converted into an electrical acoustic signal. In the signal processing unit 2, the acoustic signal undergoes the waveform shaping processing effected by the input signal shaping circuit 21 for the purpose of noise elimination. The acoustic signal outputted from the signal input shaping circuit 21 is then inputted to the frequency counter 22 to be converted into a corresponding digital signal, from which frequency data is generated by counting the pulses contained in the digital signal.
The frequency data is then processed by the CPU 23 to a form suited for a numerical display to be subsequently displayed on the display device 24. In this conjunction, it will readily be appreciated that the technique for measuring the natural vibration frequency of the of the string of a musical instrument can be applied to measurement of the natural vibration of a belt employed in the engine for the motor vehicle by collecting the vibration sound generated by the belt by applying a vibration and collecting it by the microphone 1 and processing the acoustic signal by the signal processing unit 2 for displaying the natural vibration frequency of the belt and/or the tension arithmetically determined on the natural vibration frequency in terms of a numerical value.
However, because the hitherto known belt tension measuring method for estimating the tension of the basis of a relation between the deflection of the belt as brought about by applying a pressure by means of the pressure gauge is essentially of mechanical nature, it is necessarily required that there has to be available a relatively large space for accommodating or installing the pressure gauge as well as an instrument for measuring the deflection. In other words, the hitherto known method is imposed with spatial limitation and thus encounters great difficulty in carrying out the tension measurement in the environment where the available space is restricted. Additionally, the mechanical tension measuring method known heretofore suffers a problem that the tension as measured is susceptible to error involved due to error in reading the values as measured.
Furthermore, in conjunction with the procedure for determining the tension of the belt employed in the motor vehicle by vibrating it for reading the natural vibration frequency thereof, it is noted that the natural vibration frequency of belts may vary from one to another belt in dependence on the sizes of the belts as well as the span length thereof even when the belts are made of a same material. Consequently, there has been required a frequency-to-tension conversion look-up or reference table containing proportional constants for conversion of the natural vibration frequency to the tension by taking into account the sizes of the belts, the states in which the belts are spanned between or among the pulleys and other conditions. To say in another way, the belt spanning condition may differ from one to another motor vehicle. Thus, because the frequency-to-tension conversion table has to be renewed or updated every time the belt spanning condition or state varies or changes, there arises an additional problem that the belt tension measurement can not be effected with reliability immediately after the exchange of the belt with a fresh one.